Project Summary Mammalian chromosome ends or telomeres are tightly regulated by the telomerase that mediates telomere elongation and telomere-binding proteins that cap and protect telomere ends. Telomere DNA normally adopts a closed conformation, capped and protected by a multitude of telomere-binding proteins, to prevent DNA damage and genome instability. Telomeres become open and linear during DNA replication to enable telomerase access for telomere elongation. Exposed and critically short telomeres, as a result of mutations in telomerase and telomere regulators, also become open and susceptible to damage and genome instability, ultimately leading to cancer. Mutations in telomere-binding proteins and the TERT promoter have been identified in a number of cancers. Most cancer cells have up-regulated telomerase expression and activities, and cancer cells appear highly sensitive to perturbations in telomerase activities and telomere capping, making the telomerase attractive for therapeutic targeting. A comprehensive study of telomerase regulators therefore should greatly facilitate our understanding of telomerase dysregulation in cancer and the discovery of new drug targets. We have developed an arrayed whole-genome protein interaction network screening strategy based on the Bi-molecular Fluorescence Protein Complementation (BiFC) assay. A pilot TERT BiFC screen identified several proteins as key components of the telomerase complex, including a protein we named TARP1 that has never been characterized before. We propose here to screen genome wide for cell cycle-dependent regulators of the telomerase, and to examine the mechanisms and function of the TARP1-telomerase complex. We will use inducible TARP1 knockout cells generated by CRISPR/Cas9 as well as mouse xenograft models for these studies. Our work will have important implications in devising effective treatment strategies for cancers that result from telomere dysfunction induced genome instability.